Contents

Measurement

Sea level measurements from 23 long
tide gauge records in geologically stable environments show a rise of around 200 millimetres (7.9 in) during the 20th century (2 mm/year).

Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level.[3] Sea level varies quite a lot on several scales of time and space. This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature,
salinity and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a
datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point.

To an operator of a
tide gauge, MSL means the "still water level"—the level of the sea with motions such as
wind waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the
tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in MSL can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates.

Since the times of the
Russian Empire, in
Russia and other former its parts, now independent states, the sea level is measured from the zero level of
Kronstadt Sea-Gauge.

In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 1.230m below the average sea level.

In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level. Elsewhere in Europe vertical elevation references (European Vertical Reference System) are made to the
Amsterdam Peil elevation, which dates back to the 1690s.

Height above mean sea level

Height above mean sea level (AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to the average sea level datum. It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (MSL) (contrast with
flight level), and in the
atmospheric sciences, and
land surveying. An alternative is to base height measurements on an
ellipsoid of the entire Earth, which is what systems such as
GPS do. In aviation, the ellipsoid known as
World Geodetic System 84 is increasingly used to define heights; however, differences up to 100 metres (328 feet) exist between this ellipsoid height and mean tidal height. The alternative is to use a
geoid-based vertical
datum such as
NAVD88.

When referring to
geographic features such as mountains on a
topographic map, variations in elevation are shown by
contour lines. The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both.

In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see
Amsterdam Airport Schiphol.

Difficulties in use

To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's
gravitational field. In reality, due to currents, air pressure variations, temperature and salinity variations, etc., this does not occur, not even as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as (stationary)
ocean surface topography. It varies globally in a range of ±2m.

Historically, adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month
Metonic cycle and the 223-month
eclipse cycle on the tides.[5]

Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps. The term "steric" refers to global changes in sea level due to
thermal expansion and
salinity variations. The term "isostatic" refers to changes in the level of the land relative to a fixed point in the earth, possibly due to thermal buoyancy or
tectonic effects; it implies no change in the volume of water in the oceans. The melting of
glaciers at the end of
ice ages is one example of eustatic
sea level rise. The
subsidence of land due to the withdrawal of
groundwater is an isostatic cause of relative sea level rise.
Paleoclimatologists can track sea level by examining the rocks deposited along coasts that are very tectonically stable, like the east coast of North America. Areas like volcanic islands are experiencing relative sea level rise as a result of isostatic cooling of the rock which causes the land to sink.

On other planets that lack a liquid ocean,
planetologists can calculate a "mean altitude" by averaging the heights of all points on the surface. This altitude, sometimes referred to as a "sea level", serves equivalently as a reference for the height of planetary features.

Change

Local and eustatic

Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by
waves and
tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as
sea level changes. Some land movements occur because of
isostatic adjustment of the
mantle to the melting of
ice sheets at the end of the
last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the
land slowly rebounds. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the
geoid and
true polar wander.
Atmospheric pressure,
ocean currents and local ocean temperature changes can affect LMSL as well.

Eustatic change (as opposed to local change) results in an alteration to the global sea levels due to changes in either the volume of water in the world's oceans or net changes in the volume of the
ocean basins.[6]

Short term and periodic changes

Melting glaciers can cause a change in sea level

There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level. Two major mechanisms are causing sea level to rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands.[7]

Recent changes

For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm (0.07 in) per year.[8] Most of this rise can be attributed to the increase in temperature of the sea and the resulting slight
thermal expansion of the upper 500 metres (1,640 feet) of sea water. Additional contributions, as much as one-quarter of the total, come from water sources on land, such as melting snow and
glaciers and extraction of groundwater for irrigation and other agricultural and human uses.[9]

Aviation

Pilots can estimate height above sea level with an
altimeter set to a defined
barometric pressure. Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either
QNH or "altimeter" and is transmitted to the pilot by radio from
air traffic control (ATC) or an
automatic terminal information service (ATIS). Since the terrain elevation is also referenced to MSL, the pilot can estimate height above ground by subtracting the terrain altitude from the altimeter reading.
Aviation charts are divided into boxes and the maximum terrain altitude from MSL in each box is clearly indicated. Once above the transition altitude, the altimeter is set to the
international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.[10]